Silicon carbide semi-conductor igniter structure

ABSTRACT

Igniter for jet and other internal combustion engines with a hot pressed, ceramic, electrically semiconducting button disposed between a center electrode and a ground electrode to provide a path along which spark discharge occurs. The button consists essentially of particles of silicon carbide bonded together by a ceramic matrix which can be alumina, mullite, forsterite, a glass frit spinel (MgO.A1203), a mixture of magnesium oxide and cobalt oxide, zirconium silicate, silica or the like. An igniter according to the invention is peculiarly adapted for service under severe conditions in an engine having a relatively low voltage (e.g., 3,000 volts All voltages reported herein are in volts DC unless otherwise indicated. or less), high-energy (e.g. 1 to 20 joules) ignition system.

United States Patent Continuation-impart of application Ser. No. 545,593, Apr. 27, 1966, now abandoned.

[54] SILICON CARBIDE SEMI-CONDUCTOR IGNITER STRUCTURE 10 Claims, 3 Drawing Figs. [52] US. Cl 313/130, 313/131, 315/46 [51] Int. Cl ..H0lj 19/78, HOlt 13/02 [50] Field of Search 313/130,

, v v 516 [56] References Cited UNITED STATES PATENTS 2,578,754 12/1951 Smits 123/169 2,684,665 7/ l 954 Tognola 315/46 2,786,158 3/1957 Tognola 315/46 3,052,814 9/1962 Edwards etal. 313/131 3,344,304 9/ l 967 Rademacher 313/131 FOREIGN PATENTS 209,356 7/1957 Australia 313/131 Primary Examiner-John Kominski Assistant Examiner-E. R. LaRoche Attorney-K. W. Brownell ABSTRACT: lgniter for jet and other internal combustion engines with a hot pressed, ceramic, electrically semiconducting button disposed between a center electrode and a ground electrode to provide a path along which spark discharge occurs. The button consists essentially of particles of silicon carbide bonded together by a ceramic matrix which can be alumina, mullite, forsterite, a glass frit spine] (MgO-AI O a mixture of magnesium oxide and cobalt oxide, zirconium silicate, silica or the like. An igniter according to the invention is peculiarly adapted for service under severe conditions in an engine having a relatively low voltage Keg 3,000 volts* or less), high-energy (e.g., 1 to 20 joules) ignition system.

SILICON CARBIDE SEMI-CONDUCTOR IGNITER STRUCTURE This application is a continuation-in-part of application Ser. No. 545,593, filed Apr. 27, I966, now abandoned.

BACKGROUND OF THE INVENTION surface which is generally considered to be an electrical insu--- lator disposed between a center electrode tip and a ground electrode, so that spark discharge occurs generally along the resistor surface. The high applied voltage is required to ionize the spark gap and, consequently, to enable the discharge. Igniters for use in low-voltage systems have an electrically semiconducting surface between the center electrode tip and the ground electrode, so that limited current flow can occur along this surface upon application of a low voltage, this current flow causes the requisite ionization and enables a high energy spark discharge with the low applied voltage.

Various electrically semiconducting ceramic bodies have heretofore been suggested* and used in igniters for low-voltage ignition systems. Previously known semiconductor bodies and engobes have been used extensively in igniters fired by low-voltage, high-energy ignition systems, but have been found to be inadequate under severe service conditions, in particular high combustion zone temperatures encountered in many present-day engines. See, for example, U.S. Pat. Nos. 3,037,140 and 3,046,434.

THE PRESENT INVENTION The present invention is based upon the discovery that hot pressed, electrically semiconducting ceramic bodies consisting essentially of finely divided silicon carbide particles bonded together by a suitable ceramic phase have drastically improved service lives when appropriately assembled to provide the electrically semiconducting surface along which a high-energy spark discharge occurs in a low-voltage igniter.

OBJECTS OF THE INVENTION It is an object of the present invention to provide an improved igniter for jet and other internal combustion engines.

It is a further object of the invention to provide an igniter which includes hot pressed, electrically semiconducting ceramic body consisting essentially of particles of silicon carbide bonded together by a suitable ceramic phase, and wherein the ceramic electrically connects a center electrode of the igniter and a ground electrode thereof.

Other objects and advantages will be apparent from the description which follows, reference being made to the accompanying drawings, in which:

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a partially schematic view in longitudinal cross section showing the firing tip of an igniter according to the invention.

FIG. 2 is a view similar to FIG. 1, but showing the firing tip of a modified igniter according to the invention.

FIG. 3 is a view similar to FIGS. 1 and 2, but showing the firing tip of a further modified igniter according to the invention.

Referring now in more detail to FIG. I of the drawings, a low-voltage igniter comprises a shell 11 having an inwardly directed flange 12 at its lower extremity to constitute a ground electrode. In service, the shell 11 is suitably engaged with an associated engine so that the flange 12 extends into the firing chamber of the engine, and is grounded to the engine by virtue of contact between the shell 11 and the engine. The igniter 10 also includes a center electrode 13 coaxial with the shell 11, and terminating in a radially enlarged head 14 which is spaced from the inner extremity of the flange 12to provide an annular spark gap. The upper portion of the center electrode 13 is seated in an insulator 15, disposed within the shell 11. A semiconductive ceramic body 16 extends annularly around the center electrode 13 from the lower extremity of the insulator 15 to the upper inwardly directed surface of the flange 12. Both the flange 12 and the head 14 of the center electrode 13 are in electrical contact with a lower surface 17 of the body 16, so that current flow along the surface 17 ionizes adjacent air and enables a high-energy low-voltage spark to occur, as discussed above.

Referring now to FIG. 2, a low-voltage igniter 18 comprises a shell 19 having an inwardly directed flange 20 at its lower extremity to constitute a ground electrode. The igniter 18 also includes a center electrode 21 which extends into spark gap relationship with an opposed surface of the flange 20. The upper portion of the center electrode 21 is seated in an insulator 22, disposed within the shell 19. A semiconductive ceramic body 23 extends annularly around the center electrode 21 from the lower extremity of the insulator 22 to the upper inwardly directed surface of the flange 20. An annular surface 24 of the body 23 is adjacent the gap between the center electrode 21 and an opposed surface 25 of the flange 20, so that current flow along the surface 24 enables the desired spark discharge. In the igniter 18, however, electrical contact is between a central bore of the body 23 and the electrode 21, and between an upwardly directed frustoconical surface of the flange 20 and a mating surface of the body 23.

Referring now to FIG. 3, an igniter 27, except for dimensions, is substantially identical with the igniter 18, comprising a shell 28 having an inwardly directed flange 29 at its lower extremity to constitute a ground electrode. The igniter 27 also includes a center electrode 30 coaxial with the shell 28, and extending into spark gap relationship with an opposed surface of the flange 29. The upper portion of the center electrode 30 is seated in an insulator 31, disposed within the shell 28. A semiconductive ceramic body 32 extends annularly around the center electrode 30 from the lower extremity of the insulator 31 to the upper inwardly directed surface of the flange 12, which surface, as in the igniter 18, is frustoconical in shape. As in the igniter 18, current flow to enable the spark discharge is along an annular surface 33, while contact is between a central bore and the electrode 30, and between an upper, frustoconical surface 34 of the flange 29 and a mating surface of the body 32.

The following examples illustrate the production of ceramic bodies comprising particles of silicon carbide which are useful in the preparation of high-energy igniters of the low-voltage type, and are to be construed as illustrating and disclosing, but in no way as limiting, the invention.

EXAMPLE 1 A raw mix comprising 50 parts* of fine calcined alumina powder and 50 parts of silicon carbide powder was placed in a tumbling mill and mixed for about 30 minutes to thoroughly blend the powders. The alumina powder had an analysis as follows:

Percent A1 0 99. 0 Na O 50 F8203 SiOz 025 Loss on ignition 50 The silicon carbide powder had the following analysis:

Percent SiC 98. 2 Free 0 26 Surface silicon 01 S102 1. 50

The terms percent" and 'parts, as used herein and in the appended claims, refer to percent and parts by weight, unless otherwise indicated.

Two to 3 percent of a temporary binder was then added to the blended powders. The temporary binder consisted essentially of two parts polyvinyl alcohol and 90 parts water. Although any temporary binder may be used, it is preferred that such a binder be fugitive, that is, like polyvinyl alcohol, that it volatilize during hot pressing leaving substantially no residue. The temporary binder is used in this example to prevent segregation of the mix when placed in the mold; however, in many instances the use of a temporary binder is not required Pressed bodies measuring 1 inch diameter by l /4 inch to l k inch in thickness were pressed from the mix in the followmg manner:

The raw mix was placed in a graphite mold and subjected to a pressure on the order of 500 to 600 p.s.i. The loaded mold was then placed in a drying oven set at 1 C. for a period of 12 to 16 hours to remove excess moisture. Upon completion of the drying step the loaded mold was placed in an induction furnace and held for a period of about 16 minutes at a temperature of about l,750C. under a pressure of 4,000 p.s.i. The time of heating is, of course, dependent on the size of the article to be pressed. An atmosphere of argon was maintained throughout the hot pressing operation and subsequent cooling. Then the mold was removed from the furnace and the pressed body was ejected from the mold.

The pressed bodies obtained had porosities of about 12 percent as determined by ASTM C-46 and were machined for test into annular discs about 0.12 inch thick and having outside diameters of 0.350 inch, and inside diameters of 0.103 inch. The test pieces were then assembled with insulators, electrodes and shells into assemblies of the type shown in FIG. 1 and minimum voltage requirement for sustained sparking was determined, as well as erosion rate. The bodies were found to have a voltage gradient (voltage requirement for sustained sparking divided by the distance in centimeters between the head 14 of the center electrode and the opposed vertical surface of the flange 12) of about 19,500 volts per centimeter. Erosion rate was determined by mounting the assemblies in a fixture so that the surfaces 17 faced upwardly, and subjecting the assemblies to high energy sparking (20 joules) at the rate of 120 sparks per minute while jet fuel was dripped onto the surfaces 17. The applied voltage was 2,000. The test pieces produced as described above were found to have an erosion rate of 0.0002 gram per hour.

igniters were also produced from alumina silicon carbide bodies made as described above. The bodies were machined to shape as shown in FIG. 3: longitudinal dimension, or height, 0.250 inch, major diameter 0.382 inch, height of right cylindrical wall (H in FIG. 3) 0.091 inch, and angle of frustoconical surface (0 in FIG. 3) 22 W.

When operated experimentally, under severe conditions, in jet engines, igniters produced as described in the preceding paragraph hereof were found to have dramatically improved useful service lives by comparison with igniters wherein the surface corresponding with the surface 33 carried a presently known engobe coating to make it electrically semiconducting, and also by comparison with igniters where a previously known semiconducting body was substituted for the body 32, but, in both cases, otherwise substantially identical. The dramatically improved service life of igniters produced as described in the preceding paragraph hereof has also been demonstrated by a severe laboratory test*, and it has been found that the results of the severe test correlate well with results achieved in actual service in a jet engine. It has also been found that the results of voltage gradient and erosion rate testing, performed as described above, correlate well with results of the severe *Described below. lahor a t ory test and with results achieved by testing under actual service conditions, in operating jet engines.

The severe laboratory test to which reference has been made above involves a 40 hour endurance test conducted at 700 C. and what is called a fuel flood and quench test. The endurance test is similar to the erosion test described above,

except that it is corfducted at 700 C., using a 12 joule discharge at a rate of 60 to 70 sparks per minute, and is conducted in four 10 hour periods, for a total of 40 hours, with inspection of the igniters after each 10 hour period. The fuel flood and quench test involves a first phase, fuel flooding, during which jet engine fuel is flowed onto the surface 33 at a rate of 20 cc. per minute. Twelve joules spark discharges, 70 to 80 per minute, are used for 30 seconds and then discontinued for 30 seconds: total, 20 cycles. After the 20 cycles of the fuel flood test, the same igniters are subjected to 180 cycles of a quench test. Each cycle involves heating the tip of the igniter, while sparking at 70 to 80 sparks per minute (12 joule discharges) to about 600 C. in the direct flame of a burner, and then immersing the tip of the igniter in cool (e.g., 40 C.) jet engine fuel. The heating and sparking portion of each cycle lasts for about 30 seconds; the igniter is then inverted into cool jet engine fuel, where it remains for about 15 seconds, and is then returned to its previous position. Each cycle of the quench test involves approximately 1 minute. After the igniter has completed the 180 cycles of the quench test, it is again subjected to 20 cycles of the fuel flood test, and then to a second 180 cycles of the quench test. Each igniter is examined after each phase of the fuel flood and quench test.

The igniters produced as described above, using the identified buttons made from the semiconducting ceramic material of example 1 were shown to undergo only minor changes as a consequence of the severe laboratory test. initially, in a typical case, an applied e.m.f. of about 1,100 volts caused sustained sparking; after testing, an applied e.m.f. of about 1,200 volts sustained sparking, and only minor deterioration of the surface 33 was observed.

EXAMPLES 26 Additional electrically semiconducting ceramic bodies were made in the manner described in example 1, except that the proportions of silicon carbide and alumina were varied from 20 percent to 70 percent of silicon carbide, 80 percent to 30 percent alumina. Representative information concerning typical such bodies and the results of voltage gradient and erosion rate testing are presented in table 1, below.

TABLE I" Composition Erosion Voltage rate, SiC, A1 0 Porosity, gradient, grams] percent percent percent v./cm. hour Example:

The following example illustrates another method for producing an electrically semiconducting ceramic body from silicon carbide and alumina:

EXAMPLE 7 Silicon carbide grain analyzing about 96 percent SiC is milled for a sufficient time to insure that the average particle size is no greater than 30 microns. A mixture comprising 50 parts of the milled silicon carbide and 50 parts of the fine calcined alumina used in example 1 is thoroughly mixed to ensure a homogeneous mixture. A portion of the mixed powder is placed dry in a graphite mold and hot pressed as in example 1 to obtain a dense body. After cooling, the bodies may be machined as desired for use as described above in low voltage sparking devices.

There is sometimes a tendency for alumina grain growth during hot pressing of alumina-silicon carbide compositions. lt has been found that such grain growth can be prevented, or minimized so that it is negligible, by minor amounts of materials such as magnesium oxide, added before firing to the alumina. Information concerning typical bodies which have been produced using minor amounts of magnesium oxide for the indicated purposes is presented in table II, below.

igniter type described in this paragraph) in the previously described severe laboratory test. The bodies of examples l and 11, containing 35 percent of silicon carbide, were substantially equivalent in all respects with the example 1 materi- TABLE II Soak Erosion temper- Soak Percent of rate,

atur time, theoretical grams Minimum Magnesia F. minutes density per hour voltage 1 10.1 tlliCIOfflIB-d for sustained spark, 0.050 inch gap.

The bodies of examples 8 through 18 were produced by hot pressing in an inductively heated graphite mold. The mold was about 3 inches in diameter and had a circular bore one-half inches in diameter extending axially therethrough. Temperatures were determined optically by means of a pyrometer sited on the bottom of a radial hole extending from the exterior of the mold a depth of one-half inch; the bottom of the hole was adjacent a sample being hot pressed. Heating was controlled so that the measured temperature increased at a rate of about 100 F. per minute to the soak temperature indicated in the table, and then to maintain the soak temperature for the time indicated in table II. The indicated batch materials were pressed into cylindrical slugs about one-half inch in diameter, and inserted into the mold on top of a graphite plunger which filled the lower portion of the bore of the mold. An upper graphite plunger was then inserted above the sample, and heating was commenced, as indicated. Pressure was first applied to the sample when the indicated temperature reached approximately 2,700 F. The initial pressure applied was 600 pounds per square inch, and this pressure was increased gradually until about the time the soak temperature was reached to a maximum of 6,000 pounds per square inch. The maximum pressure was maintained throughout the soak period, and during cooling of the mold to about 2,700 P.

On the basis of extensive experimental engine testing, it has been determined that semiconducting buttons having the shape of the button 23 (FIG. 2: longitudinal dimension of height 0.250 inch, major diameter 0.345 inch, minor diameter, of the surface 24, 0.250 inch, angle 0, FIG. 2, 45 bore diameter 0.151 inch), when assembled in igniters as indicated, were substantially equivalent to the example 1 material. Some interesting trends were noted on the basis of performance of the materials of examples 8, 9, I0, 11, 14, and 15 (shape and al, but a somewhat higher voltage requirement after the severe laboratory test. The material of examples 8 and 9, containing only 20 percent of silicon carbide, required a somewhat higher voltage to sustain sparking, before and after testing. In addition this material showed a greater tendency to deteriorate than did the materials of examples 1 and 12 but significantly less tendency to deteriorate than did previously known semiconducting materials. The material of examples l5 and 16, containing percent of silicon carbide, required a significantly lower voltage to sustain sparking, before and after testing, than did the materials of examples 1 and 12, but showed somewhat more erosion after the severe laboratory test.

It will be apparent from the data reported herein that hot pressed, electrically semiconducting ceramic bodies consisting essentially of alumina and silicon carbide, and containing as little as about 20 percent of silicon carbide can be used to significant advantage in producing igniters according to the invention. Preferably, such bodies contain at least about 25 percent of silicon carbide and, most desirably, for optimum combinations of erosion resistance and voltage requirements, con-' tain at least about 30 percent of silicon carbide. Such bodies containing as much as about percent of silicon carbide can be used to produce igniters according to the invention. Preferably, the bodies contain not more than about 80 percent of silicon carbide and, most desirably, contain not more than about 75 percent of silicon carbide for an optimum combination of erosion resistance and voltage requirements.

Various other hot pressed bodies which have been produced and tested as described above for minimum voltage required to sustain sparking (0.05 inch gap) and for erosion rate are identified in table III, below. In all cases, the bodies contained 50 parts of a ceramic binder phase and 50 parts of silicon carbide.

TABLE III Soak Erosion Electrically temper- Soak Percent of rate, Minimum conducting ature, time, theoretical grams applied Ceramic binder material F. minutes density per hour voltage Example No.:

19 Spinel* (Mg0.Ai20a) 0 3,400 15 99.2 .00039 1,373 20 Mullite" (3 A0032 S102). 3,400 16 91. 0 00023 1, 700 Glass trit*** SiC 2,600 15 95.6 .00027 2, 083 22"; Forstente**** (2 MgO.SiO2) Si 3,000 15 93.2 .00030 1, 483 Magnesia*** (MgO+CoO) SiC 3,000 15 88.7 .00066 1, 443 Zircon (ZrOz.SiO2) $10 3,300 15 76.8 .00036 1,863 25 Silica (SiO2) 810 3,050 15 83.2 .00016 2,088

For sustained spark, 0.050 inch ga Composition, by percent, MgO 28.4, A: 71.6. "Composition, by percent, A1203 71.8, SiOz 28.2.

*The fruit contained, by percent, A1203 8, SiOz 50, C210 32 and MgO 10. "*Com osition, by percent, MgO 57.4 and SiOz 42.6.

""47 parts of MgO and 2% parts of C00.

It is apparent from the information presented in table Ill that various binder materials are equivalent to alumina for use in producing hot pressed ceramic bodies for igniter assemblies according to the invention. In general, any ceramic binder phase can be used provided that the phase can be hot pressed at an achievable temperature to a relatively dense condition, usually at least about 80 percent of theoretical density, and which is stable in its hot pressed condition and is sufficiently refractory for service in an internal combustion engine. Pure magnesium oxide is not a suitable binder phase because, although it can be hot pressed to an adequate density, and has adequate temperature resistance, the hot pressed material is not stable but, instead, hydrates relatively relatively rapidly. The purpose of the cobalt oxide in example 23, therefore, was to inhibit hydration. Similarly, various glasses would be unsuited because of their comparatively low-temperature resistance.

When materials other than silicon carbide and alumina are ing tip, and mounted within an insulator which, in turn, is mounted within an exterior metal shell, a ground electrode integral or in electrical contact with the shell and in spaced, spark-gap relation with the firing tip of the center electrode, and an electrically semiconducting surface adjacent the spark gap and in electrical contact with the center electrode an with the ground electrode, the improvement wherein the electrically semiconducting surface is a surface of a ceramic body hot pressed to a density at least about 80 percent of theory and consisting essentially of silicon carbide particles, bonded into said ceramic body by a stable, ceramic binder phase selected from the group consisting of alumina, a spinel of the formula MgO.Al2O mullite, magnesium silicates, magnesia stabilized against hydration, and silica, the volume ratio of said particles to said binder being at least as high as that when said body contains percent by weight of silicon carbide and 80 percent by weight of alumina, and not higher than that when I said body contains 90 percent by Weight of silicon carbide used in producing semiconducting bodies for use in igniters 20 and 0 Percent by Weight 3 E 1}??- according to the invention, the amounts of binder material and conductive material preferably are such that the volume I ratios of one to the other are substantially the same as the corresponding volume ratios when alumina and silicon carbide are employed in the weight percentages discussed above as operable, preferred and optimum.

Erosion rates, measured as described above, give an estimate of the service lives of the bodies involved under engine conditions. The erosion rate of bodies made in accordance with this invention is preferably less than about 0.0004 gm./hr.; however, erosion rates of up to 0.0015 gm./hr. are not objectionable for some types of sparking devices, and even higher erosion rates can be tolerated, particularly in engines having ignition systems which operate at less than 20 joules, and if the compositions have other advantages. For example, the high erosion rates of the bodies of examples l5 and 16 were more than ofiset by the advantageous voltage requirements, as determined before and after the severe laboratory test.

It will be apparent that various changes and modifications can be made from the specific details of the invention as set forth herein without departing from the spirit and scope of the attached claims.

Iclaim:

1. In an igniter for jet and other internal combustion engines, which igniter comprises a center electrode having a fir- 2. In an igniter as claim 1, the improvement wherein the ceramic binder phase is silica.

3. In an igniter as claimed in claim 1 the improvement wherein the ceramic binder phase consists essentially of alumina.

4. In an igniter as claimed in claim 1, the improvement wherein the ceramic binder phase is a magnesium silicate.

6. In an igniter as claimed in claim 1, the improvement wherein the ceramic binder phase is mullite.

7. In an igniter as claimed in claim 1, the improvement wherein the ceramic binder phase is magnesia, stabilized against hydration.

8. In an igniter as claimed in claim 4, the improvement wherein the magnesium silicate is a calcium magnesium aluminum silicate.

9. In an igniter as claimed in claim 3, the improvement wherein alumina constitutes from 25 to 70 percent of the ceramic body, and silicon carbide from to 30 percent.

10. In an igniter as claimed in claim 4, the improvement wherein the magnesium silicate is forsterite. 

2. In an igniter as claim 1, the improvement wherein the ceramic binder phase is silica.
 3. In an igniter as claimed in claim 1, the improvement wherein the ceramic binder phase consists essentially of alumina.
 4. In an igniter as claimed in claim 1, the improvement wherein the ceramic binder phase is a magnesium silicate.
 5. In an igniter as claimed in claim 1, the improvement wherein the ceramic binder phase is a spinel of the formula MgO.Al2O3.
 6. In an igniter as claimed in claim 1, the improvement wherein the ceramic binder phase is mullite.
 7. In an igniter as claimed in claim 1, the improvement wherein the ceramic binder phase is magnesia, stabilized against hydration.
 8. In an igniter as claimed in claim 4, the improvement wherein the magnesium silicate is a calcium magnesium aluminum silicate.
 9. In an igniter as claimed in claim 3, the improvement wherein alumina constitutes from 25 to 70 percent of the ceramic body, and silicon carbide from 75 to 30 percent.
 10. In an igniter as claimed in claim 4, the improvement wherein the magnesium silicate is forsterite. 